The objectives of the research program proposed here are fourfold: 1. An analysis at the electron microscopic (EM) level of synaptogenesis in the chick optic tectum during normal embryonic development using EM autoradiography (EMAR). The data will be used to test the predictions of two hypotheses of neural specificity: Do axons form connections with the first neurons they encounter during development, or do they show a more selective pattern of synapse formation? 2. Light microscopic (LM) and EM studies of the normal morphology of two visual centers, the ectomammillary nucleus and the ventral lateral geniculate nucleus, which show remarkably different responses to deafferentation during embryonic development. Subsequent experimental studies will analyze the changes taking place in both of these nuclei following optic-cup ablation in order to further investigate the importance of morphological factors in the maintenance of nerve cells by their afferent input during development. 3. LM studies on the visual system of a common laboratory mammal, the golden hamster (Mesocricetulus auratus), using tritiated thymidine autoradiography to determine the time of cell origin and quantitative autoradiographic tracing methods to determine the time of arrival of afferents to the central visual nuclei; these studies will provide a baseline for subsequent experimental work on the development of the mammalian visual system. 4. Techniques will be developed for complete and partial retinal ablation in mammalian (hamster) fetuses. These LM studies on the neuronal specifity of the mammalian visual system will help to determine the degree of correspondence between the pattern of development of the mammalian visual system and that analyzed in the previous studies employing a lower vertebrate.